1. Field
This document relates to a polymer dispersed liquid crystal (PDLC) display and a method of fabricating the same.
2. Related Art
In general, a liquid crystal display (LCD) is an apparatus for representing images by employing the optical anisotropy and birefrigence properties of liquid crystal molecules, and generally employs a thin film transistor (TFT) LCD using a TFT as a switching element.
However, in a general LCD, a polarized plate must be used, and about 70% to 80% of a total amount of light incident from a backlight unit is substantially lost and then displayed to the outside. For this reason, the LCD is problematic in that it has low optical efficiency and has problems, such as optical scattering and/or phase distortion due to an orientation process.
To solve the problems, a PDLC display has been proposed. PDLC is one of liquid crystal cells used in a LCD, and is characteristic in that the transmittance of light is controlled according to the scattering intensity and the polarized plate is not required. There have been proposed several kinds of PDLC structures, such as one in which a plurality of liquid crystal molecule particles of several mm in size are dispersed in a polymer material and one in which liquid crystals are comprised in a net-shape polymer. That is, the PDLC display is a device in which the liquid crystal material and materials other than the liquid crystals are dispersed irregularly and it is operated using the difference in the refractive index between the two materials.
If a voltage is not applied to the PDLC display, the direction of the liquid crystal molecules becomes irregular and scattering is generated at the interface in which the refractive index of the liquid crystal molecules is different from that of a medium. However, if a voltage is applied to the PDLC display, the direction of the liquid crystal molecules is orderly arranged and it results in a transmission state because the refractive index of the liquid crystal molecules is identical to that of the medium. At this time, if the thickness of the liquid crystal cell is not large enough, sufficient luminance is not secured, leading to a high driving voltage.
The PDLC display can be fabricated easily because it does not require an orientation film and a polarized plate and has flexibility. Further, the PDLC display can be used for large-sized display devices and projection TV because of an excellent luminance characteristic.
FIG. 1 is a schematic construction of a general PDLC display.
Referring to FIG. 1, a general PDLC display comprises a PDLC 10, a backlight unit 20, and a reflection plate 30. The PDLC 10 comprises a polymer matrix 11 and micro liquid crystal droplets 12. Liquid crystal molecule particles 13 are intervened in the micro liquid crystal droplets 12 in a capsule shape.
In the PDLC 10, in an off state (that is, when a voltage is not applied), micro liquid crystal droplets are arranged randomly and therefore light is scattered because of the refraction between the micro liquid crystal droplets 12 and the polymer matrix 11, thereby implementing a dark state. However, in an on state (that is, when a voltage is applied), the micro liquid crystal droplets are arranged in a row and therefore the difference in the refractive index between the micro liquid crystal droplets 12 and the polymer matrix 11 is reduced, so a liquid crystal cell transmits light, thereby implementing a white state.
A detailed method of implementing the dark state and the white state of the PDLC display is described below.
FIGS. 2a and 2b are views illustrating a detailed operation principle of the PDLC display of FIG. 1.
Referring to FIGS. 2a and 2b, the PDLC display comprises a rear substrate 14 equipped with a first driving electrode 16, and a front substrate 15 equipped with a second driving electrode 17. The rear substrate 14 and the front substrate 15 are opposite apart from each other at a predetermined distance. A polymer matrix 11 in which nematic liquid crystal molecule particles 13 are arranged within respective micro liquid crystal droplets 12 is intervened between the rear substrate 14 and the front substrate 15.
In the conventional PDLC display constructed as above, when a voltage is not applied between the first driving electrode 16 and the second driving electrode 17, incident light is all scattered because of the difference in the refractive index between the micro liquid crystal droplets 12 and the polymer matrix 11 as shown in FIG. 2a, thereby implementing the dark state. In other words, as shown in FIG. 2a, before an electric field is formed between the first and second driving electrodes 16 and 17, the liquid crystal molecule particles 13 within the micro liquid crystal droplets 12 are arranged randomly because of the absence of the orientation film. Accordingly, incident light is scattered while passing through the polymer matrix 11, thereby implementing the dark state.
On the other hand, if a power supply unit 18 applies a voltage between the first driving electrode 16 and the second driving electrode 17, the liquid crystal molecule particles 13 within the micro liquid crystal droplets 12 have their long axes arranged parallel to an electric field E, as shown in FIG. 2b. Thus, incident light transmits the polymer matrix 11, thus implementing the white state. That is to say, when the electric field E is formed between the first and second driving electrodes 16, 17, the liquid crystal molecule particles 13 within the micro liquid crystal droplets 12 have their long axes arranged parallel to the electric field E. As the liquid crystal molecule particles 13 are arranged parallel to the electric field E in one direction, the white state is implemented.
The conventional PDLC display has a high transmission and a perfect visual field characteristic, but is limited that the dark state is implemented by only random scattering of light by micro liquid crystal (LC) droplets arranged in a random direction, which exist within a polymer. Further, the conventional PDLC display requires a little high driving voltage and thus requires the development of a liquid crystal material and process for solving the high driving voltage.
Meanwhile, the conventional LCD and PDLC displays must use a backlight unit for providing a light source and also requires a reflection plate and a spreading plate. Due to this, there are problems in which the thickness of the display is thick and the manufacturing cost is increased.